Does the delay happen in Firefox safe mode? It could be an extension causing the delay.

I am not aware that Firefox has a safe mode. (How do you enable/start it?)

The problem seems to happen most often if I have visited a lot of sites/pages during the current session or since the last time I cleared my browser history. I'm not sure I would want to do that much surfing without the protection of some of my extensions enabled.

Thanks. I appreciate your reply.

http://kb.mozillazine.org/Safe_mode

There is more than one way to start safe mode, but the easiest way these days is to use Help > Restart with add-ons disabled.

Are you doing that via History > Clear Recent History ?

• https://support.mozilla.org/kb/Clear+Recent+History

Are you only clearing the history or are you clearing more items (cookies, cache)?
If you clear more items then try to select only one of them to see if you can isolate the cause.

You can check for problems with the places.sqlite database file in the Firefox Profile Folder.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file